Russia carried out its largest air strike on Ukraine's energy system since its invasion in February 2022, damaging power units at a dam and causing blackouts for more than a million people
A territorial row over the islands, which the Soviets seized at the end of World War Two, has prevented the two countries from signing a formal peace treaty. Japan calls the islands the Northern Territories and Russia calls them the Kurils.
Mishustin visited the islands on Monday, prompting a top Tokyo government spokesman to say Japan would protest to Russia over the visit.
Mishustin said he would discuss an "unprecedented" set of economic measures aimed at the islands' development with President Vladimir Putin.
"This would be interesting... for Japan as well which could create jobs and work with you if it is interested," he said, addressing local businessmen, according to a RIA report.